Non Destructive Testing - NTD of Welds
Introduction to Nondestructive Testing of Welding.
When I served my time in Harland and Wolff shipyard in Belfast in the 1960’s, welding had thankfully taken over from riveting. My old next door neighbour had been a riveter on the Titanic and was stone deaf due to the constant hammering of the pneumatic equipment used in those days.
Today most shipyards fabricate large components of hull and deck plates welding them together using automatic welding techniques, which are very efficient and reliable. However, due to stringent regulations set by the various authorities these welds require inspection which nowadays consists of nondestructive testing, known as NDT. There are various types of NDT which allow ferrous and nonferrous welding to be inspected for cracks or other flaws.
This is an article on welding inspection and covers the different methods employed in engineering. We begin by examining some pertinent welding techniques, following on with the methods of NDT.
Welding Techniques used in Marine and Petro-chemical Industries
Before we examine the different types of NDT, it will be worthwhile to give a brief overview of welding alloys of lined pipe and the requirements for pre- and post-welding heat treatment.
In the offshore hydrocarbon and petrochemical industry, there are many different alloys used in pipe fabrication which require specialist welding using Tungsten Inert Gas (TIG) welding that uses an inert gas to shield the weld area from air contact. The gas is supplied through the welding nozzle which holds a tungsten rod. The rod fuses the edges of the two components; a filler rod can also be used if required.
Metal Inert Gas (MIG) welding also uses gas shielding, but in this process as well as supplying gas, the nozzle also supplies a filler rod continuously, which is used to weld the two metals together.
Pre- and post-weld heat treatment
In some heavy-wall pipe and structural components, where metal arc welding is used, heat pads are applied to each side of weld area to relieve any stresses in the metals before welding. This is repeated immediately after welding is completed, the timing and temperatures involved in the heat treatment being calculated and controlled by the welding engineer and his supervisors.
Categories of Non-destructive Testing
The different types of non-destructive testing used to inspect welding are shown below:
- Radiographic Inspection (Graphs)
- Magnetic Particle Inspection(MPI)
- Ultrasonic Testing (UT)
- Dye Penetration (Dye Pen)
Description and Application of Non Destructive Testing Methods
Radiographic Inspection (Graphs)
This is carried out where the welded components require a very critical inspection technique due to their application.
Shielding from x-ray and gamma-ray radiation is a strict requirement; this can be of a portable means or the components can be brought to a specialised building to be x-rayed. In my old offshore construction yard, radiography was carried out during the dead hours between shifts, when a visible and audio alarm would howl continuously to warn against entrance to the assembly buildings.
Anyway, the relative components are exposed to the radioactive source from which a radiograph is produced. This will show any irregularities in the welding when checked by an experienced radiograph interpreter.
Magnetic Particle Inspection (MPI)
The welded area is coated with a magnetic flux containing iron ferrous particles. An electric yoke magnet is placed across the flux producing a visible magnetic field. Any surface cracks or weld irregularities will become evident by the accumulation of the flux due to the forming of a new magnetic pole each side of the crack. This is a very inexpensive and quick method of weld inspection, however it is only used to check fillet welds for surface imperfections, and of course can only be used on ferrous metals.Ultrasonic Testing (UT)
This method can detect surface and internal irregularities in ferrous and non-ferrous metal welding.
It operates by transmitting high frequency pulsing sound waves through the weld, the results being transmitted to a monitor as a trace. If the pulse comes in contact with an irregularity in the weld, the waves are sent back to the transmitter and show up on the monitor screen. The defect can be placed very accurately, but it requires an experienced operator to interpret the tracings on the monitor.
Ultrasonic Testing (UT)
This method can detect surface and internal irregularities in ferrous and non-ferrous metal welding.
It operates by transmitting high frequency pulsing sound waves through the weld, the results being transmitted to a monitor as a trace. If the pulse comes in contact with an irregularity in the weld, the waves are sent back to the transmitter and show up on the monitor screen. The defect can be placed very accurately, but it requires an experienced operator to interpret the tracings on the monitor.
Dye Penetration (Dye Pen)
This system operates on a capillary action principle where a fluid in the form of a florescent or nonflorescent dye is applied to a weld surface.
Once the fluid has been given time to penetrate the surface, (between 15 and 30 minutes) the excess is wiped away and a developing fluid applied. The developer draws fluid out from any flaws and when viewed under a UV or white light, imperfections in the weld become visible.
Internet Sites Visited
https://www.engineersedge.com/inspection/marnetic_particle.htm
